Saws, drills, and particularly hammer drills, when used to drill or cut into walls, may inadvertently contact buried or concealed cables, conduits, pipes, or nails, or, also, reinforcing rods which may be inserted in cement walls. If a drill bit or saw blade--hereinafter for short "bit"--meets one of these obstructions within a wall, it is always possible that the conduits or pipes will be damaged, for example by being cut or having a hole drilled thereinto. Even if the armoring or metal used for the concealed conduits, pipes and the like is sufficiently sturdy to deflect the bit, or hard enough so as not to be damaged thereby, damage to the bit itself is hardly avoidable. When sawing with a cross-cut blade, and particularly with a fine-tooth blade, across wood, and a nail is buried in the wood, the saw blade will be severely damaged, and may be entirely destroyed. It was, heretofore, necessary for the operator to be careful and immediately disconnect the tool as soon as contact with a buried object having a hardness characteristic which was not intended to be cut was detected. Skill and judgment is required on part of the operator to notice such contact.